Along with increase in demand for optical communication in recent years, an optical communication system in smaller size with lower cost is strongly required to be provided. To construct an optical communication system with high reliability and low cost, it is important that the system can transmit data for a long distance without relay. As a way to perform it, an optical fiber amplifier and an optical semiconductor amplifier have been known. In the former case, an Er doped optical fiber is excited by an LD module for excitation. However, it needs a very long Er doped optical fiber and a very large LD module for excitation, so that it is disadvantageous in that it becomes expensive, and a system tends to be large. On the other hand, the latter, i.e. the optical semiconductor amplifier, has the same structure as a semiconductor laser element, so that the latter is advantageous to a system to be downsized, in addition, the latter enables a system to be constructed in lower cost comparing to the former case.
FIG. 21 is a configuration diagram showing a related optical transmission module using an optical semiconductor amplifier. In the present specification, a direction of an optical axis is called as a Z direction, a direction vertical to a main surface of a carrier to be a base is called as a Y direction, and a direction orthogonal to the Y and the Z directions is called as an X direction.
As shown in FIG. 21, an optical transmission module 290 includes a laser module 280 incorporating an optical modulator and an optical amplifier module 260.
The laser module 280 is composed of a laser element 281, a lens 282, a lens holder in a U-shape (unillustrated, see FIG. 22), an optical isolator 283, an element carrier 284, a peltier device 285, a lens 286, a ferrule 287, an optical fiber 288, a package 289, and so on.
The laser element 281 incorporates an optical modulator, and outputs a modulated optical signal (on optical signal). The lens 282 is in a structure with a transmissive glass part set in an alloy frame, and it collects optical signals outputted from the laser element 281 in a side of the optical isolator 283. The optical isolator 283 prevents the light from returning to the laser element 281. On the device carrier 284, the laser element 281, the lens 282, and the optical isolator 283 are fixed in alignment with a same optical axis. The peltier device 285 maintains a constant temperature of the laser element 281 so that the optical fiber 288 outputs the modulated optical signal stably. The lens 286 collects the optical signal transmitted through the optical isolator 283 on the optical fiber 288. The ferrule 287 fixes the optical fiber 288 on the package 289 through the lens 286. The optical fiber 288 guides the optical signal outputted from the laser element 281 to outside the package 289.
The optical amplifier module 260 is composed of an optical fiber 261, a ferrule 262, a lens 263, a lens 264, a semiconductor optical amplifying element 265, a lens 266, a lens 267, a ferrule 268, an optical fiber 269, a carrier 270, a peltier device 271, a package 272, and so on. In other words, the optical amplifier module 260 is composed of the semiconductor optical amplifying element 265 which amplifies and outputs an incident light, the optical fibers 261 and 269 for input and output, and the lenses 263, 264, 266, 267 which couple the semiconductor optical amplifying element 265 with input/output optical fibers 261, 269 with high efficiency.
The optical fiber 261 includes a splicing part (a fusion splicing part) 273 formed an edge thereof for splicing itself and the optical fiber 288, and guides the optical signal outputted from the laser module 280 to the optical amplifier module 260. The ferrule 262 fixes the optical fiber 261 on the package 272 through the lens 263. The lens 264 collects the optical signal transmitted through the lens 263 on the semiconductor optical amplifying element 265. The semiconductor optical amplifying element 265 operates on the same principle as the semiconductor laser, and produces an optical amplification effect for light injected from outside by using a gain function in a semiconductor active region depending on current injection. The lens 266 collects the optical signal amplified by the semiconductor optical amplifying element 265 on the lens 267. To align optical axes of the lens 264 and the semiconductor optical amplifying element 265, as well as optical axes of the semiconductor optical amplifying element 265 and the lens 266, the lenses 264 and 266 are fixed on the carrier 270 through U-shaped lens holders (unillustrated, refer to FIG. 22). The ferrule 268 fixes the optical fiber 269 on the package 272 through the lens 267. The optical fiber 269 guides the optical signal amplified by the semiconductor optical amplifying element 265 to outside the package 272.
To achieve a long-distance transmission without relay, the optical signal outputted from the laser element 281 is not sufficient. Therefore, in the optical transmission module 290, the optical signal from the laser element 281 is amplified by current injection at the semiconductor optical amplifying element 265 so as to be high-power output light, and is outputted from the optical fiber 269. The laser module 280 and the optical amplifier module 260 are produced separately, and then the optical fiber 288 of the laser module 280 and an input side of the optical fiber 261 of the optical amplifier module 260 are connected by fusion splicing to be used.
In the optical transmission module shown in FIG. 21, the lenses 264, 266, 282 and the like are fixed on the carriers by laser welding (YAG laser welding) using the U-shaped lens holder so that the optical axes thereof are aligned to the optical axis of the semiconductor optical amplifying element 265 and the laser element 281, and that a focal point is placed on a prescribed position. FIG. 22 is an exploded perspective view describing a method for fixing the lens 282 using the lens holder. The lenses 264 and 266 are also welded and fixed in the same way, so that the fixing method only for the lens 282 will be presented hereinafter.
A lens holder 291 includes a plate-like base part 291a and a pair of holding plates 291b and 291c provided vertically to the platy base part 291a. An interval between the holding plates 291b and 291c is designed to be almost equivalent to a width of the lens 282. Accordingly, the lens 282 can be held in between the holding plates 291b and 291c. The lens 282 is adjusted and fixed as follows. The lens holder 291 in which the lens 282 is set is placed on the carrier 284 on which the laser element 281 is bonded by die bonding. Then, while causing the laser element 281 to emit light, the lens and the lens holder are moved in the three axes directions for adjusting the lens 282 in an optimal position.
After the adjustment, the lens holder 291 and the carrier 284 are welded by laser welding at plural points (at 4 points, for example), so that the lens holder 291 is fixed. Accordingly, the position of the lens 282 in the X direction is fixed. Next, the lens 282 is moved again, upward and downward (in the Y direction), and backward and forward (in the Z direction) with respect to the lens holder 291, for adjusting the lens 282 in an optical position in the Y direction and the Z direction, and then the lens 282 and the lens holder 291 are welded by laser welding at plural points (at 4 points, for example). As described, because the lens holder 291 and the lens 282 are combined together, the lens 282 can be adjusted and fixed optimally in a position to which the laser element 281 emits light with respect to the three axes of X, Y, Z.
However, this related optical transmission module (hereinafter, referred to as a separated optical transmission module) is composed of a laser module and an optical amplifier module in separate packages, so that each of the packages, the input/output optical fibers, and the splice part of the optical fibers require a certain area and volume to be housed respectively. Therefore, the module is limited in downsizing and it is disadvantageous in that the module becomes large in size.
So, housing the laser element and the semiconductor optical amplifying element in a same package is proposed (refer to in Patent Documents 1 and 2, for example). In an optical transmission module proposed by those Patent Documents (hereinafter, an integrated optical transmission module), a laser element and a semiconductor optical amplifying element are bonded on a carrier by die bonding, and a plurality of lenses is fixed on the carrier using U-shaped lens holders respectively. Alternatively, a laser element is bonded on a carrier by die bonding, and a small carrier on which a semiconductor optical amplifying element is bonded by die bonding is fixed on the carrier using a U-shaped holder, and also a plurality of lenses is fixed on the carrier using U-shaped lens holders respectively.    Patent Document 1: Japanese Patent Application Laid-open No. 2005-17839    Patent Document 2: Japanese Patent Application Laid-open No. 2005-19820